1. Field of the Invention
The present invention relates generally to the field of laser technology, and more particularly, to methods and apparatus for generating high energy, ultrashort optical pulses.
2. State of the Art
Both semiconductor sources (e.g., diode) and fiber sources are known which can produce ultrashort energy pulses having sub-picosecond pulse durations. Although these energy sources can provide reliable, robust operation in a compact, cost-effective manner, their inability to produce pulse energies comparable to those of large frame solid-state sources has limited their practical use. A document co-authored by the present inventor and entitled "Generation of femtosecond optical pulses with nanoJoule energy from a diode laser and fiber based system," by A. Galvanauskas et al., Appl. Phys. Lett. 63 (13), Sep. 27, 1993, 1993: Amer. Inst. of Physics, pp. 1742-1744, describes using a tunable distributed Bragg-reflector (DBR) diode laser to produce chirped pulses. The chirped pulses are compressed to sub-picosecond duration and then amplified in an erbium doped fiber amplifier (EDFA) to a level of 2 nanoJoules. However, strong nonlinear interaction in the fiber causes pulse distortion, thereby reducing attainable energy.
That is, the output energy which can be extracted from rare-earth doped fiber amplifiers, although potentially high, is limited by the amount of peak power they can sustain before causing nonlinear effects and pulse break-up. For example, single-mode erbium doped fiber amplifiers have saturation energies of approximately one microJoule; and a multi-mode erbium doped fiber amplifier as described in a document entitled "111 kW (0.5 mJ) pulse amplification at 1.5 .mu.m using a gated cascade of three erbium doped fiber amplifiers," by B. Desthieux et al., Appl. Phys. Lett. 63 (5), Aug. 2, 1993: Amer. Inst. of Physics, pages 586-587, can produce pulse energy as high as 0.5 milliJoules. However, low peak power must be maintained in a fiber amplifier, as such energy levels can render peak power of the amplified ultrashort pulses unacceptably high for a fiber (e.g., approximately 1 MegaWatt for a 1 picosecond pulse in a single-mode fiber). The light associated with this power, when confined in the small core of a fiber, results in high peak intensities which can lead to nonlinear effects and pulse break-up.
One approach for maintaining low peak power in an amplifier has been to use chirped pulse amplification to stretch the pulses prior to amplification as described in a document entitled "Compression of Amplified Chirped Optical Pulses" by Donna Strickland and Gerard Mourou, Elsevier Science Publishers B.V.: Optics Communications, Vol. 56, No. 3, Dec. 1, 1985. As described therein, ultrashort pulses from a mode-locked solid-state laser are stretched in duration using an optical fiber. Afterwards, the stretched pulses are amplified and then compressed using a double-grating compressor. By amplifying the stretched pulses of relatively long pulse duration, peak power is maintained relatively low in the amplifier such that non-linear effects and pulse break-up are prevented. In a document entitled "Generation of Ultra High Peak Power Pulses By Chirped Pulse Amplification", by P. Maine et al, IEEE Journal of Quantum Electronics, Vol. 24, No. 2, Feb. 1988, a similar chirped pulse amplification technique is described wherein a diffraction-grating pair is used in place of an optical fiber to stretch the chirped pulses.
The chirped pulse amplification technique has also been applied to fiber amplifiers using a mode-locked fiber laser source of ultrashort pulses, as described in the following three documents: (1) "Generation of High Power Ultrashort Pulses in Erbium Oscillator Power Amplifier Systems" by M. L. Stock et al, Optical Society of America Topical Meeting on Non-Linear Guided Wave Phenomenon, Cambridge, 1993, Paper PD 5; (2) a document entitled "High-Power Chirped Pulse Amplification of Femtosecond Optical Pulses in a Diode-Pumped Fiber Laser and Amplifier System" by A. Galvanauskas et al available from IMRA America, Ind., Ann Arbor, Mich.; and (3) "All-Fiber Source of 100 nJ sub-picosecond Pulses" by M. E. Fermann et al, Appl. Phys. Lett., Vol. 64, No. 11, Mar. 14, 1994. The techniques described in these latter documents can produce pulse energies up to, for example, 100 nanoJoules with a 700 femtosecond duration.
Although systems exist to produce ultrashort pulses with increased pulse energy, such systems suffer significant drawbacks. For example, the use of chirped pulse amplification with femtosecond mode-locked fiber lasers or semiconductor lasers with external cavities requires use of bulk components, and such systems are not particularly robust or reliable. Further, mode-locked lasers are operated with high repetition frequencies, and are therefore unsuitable for use in high-energy amplification which requires relatively low pulse repetition rates. Because mode-locked lasers are operated with high repetition frequencies, complicated and expensive pulse selecting systems are used, further inhibiting overall system compactness. In addition, the use of a diffraction-grating stretcher detracts from overall system compactness and robustness.
Accordingly, it would be desirable to provide a compact system capable of producing and amplifying chirped pulses to provide high energy ultrashort pulses. Further, it would be desirable to provide a compact system which is reliable and cost effective to fabricate.